Eggshells

On my walks in city places, I've been coming across a lot of small, empty eggshell halves. So delicate-looking, like miniatures for a dollhouse, but really quite strong for their size, I'm sure. Most of the shells have been a blue-green sky color, what I've thought of since childhood as robins' eggs, though I may be wrong.

Only one I've come across was whole, a blue egg with brown speckles. I wondered if it had been pushed out of the nest by a brood parasite like a cowbird (or in Europe, their cuckoo), and now some random bird parents are raising a cowbird baby.